A device of this type has been known from European Preliminary Patent Disclosure Document No. EP-OS 0,309,069. It is often desirable in sewing technology to have the seam to be prepared automatically following the course of the edge of the cut of material being sewn, doing so at the most uniform lateral distance possible. This "edge-parallel" sewing requires a control system which adjusts the direction of the relative movement between the material being sewn and the sewing needle correspondingly on the basis of the information available on the course of the edge, either by properly dimensioning two mutually perpendicular feed vectors (e.g., in a sewing machine with an XY displacement table) or by turning the material being sewn around the needle axis (in the case of sewing machines with one-dimensional feed).
In the mass production of sewn products with edge-parallel seams, it is advantageous to enter the desired course of the seam into a memory in advance as a data set in order to subsequently control the movements of the material being sewn during sewing according to a program determined by these data. For example, the material being sewn can thus be moved past the stationary presser food along a path corresponding to the seam course stored by means of an XY displacement table. However, the edge of the material cut may deviate from the stored shape during the sewing process as a consequence of various influences. These influences are, e.g., drawing of the material being sewn during sewing, keeping together incorrectly the pieces to be sewn together, or distortion of the edge of the material cut due to clamping on the XY table. The results thus obtained are not satisfactory, because constant distance between the seam and the edge is an important requirement. Even a deviation by as little as a few tenths of one millimeter leads to an aesthetically unattractive appearance. To eliminate these defects, a control system is required which takes into account the actual position of the edge during the control of the movement of the material being sewn. If the scanning range is large enough and the response time is short, such a control system alone would be able, if desired, to control the course of the seam, without being supported by data stored in advance.
A control system for guiding the seam at a constant distance from the edge of the material being sewn requires a sensor device which monitors the material being sewn in the area of the sewing needle and supplies information on the relative position of the edge in relation to the needle. Various embodiments of such devices have been known. For example, U.S. Pat. No. 4,498,404 discloses a photoelectric sensor line, on which a straight, linear area of the surface of the material being sewn, which is located in front of the sewing needle and extends at right angles to the preferred direction of sewing, is imaged by means of a mirror and lens system. On reading the photoelectric cells of the sensor line, a shadow-casting edge of the material being sewn in the imaged area becomes noticeable as a characteristic change int he signal, so that an evaluation device will be able to determine the actual three-dimensional position of the edge point imaged in order to adjust the movement of the material being sewn so that the seam formed with remain at the specified distance from the edge of the material being sewn.
Since the area being scanned with this prior-art sensor device is limited to a stationary straight line on one side of the sewing needle, the edge guiding is able to function only as long as the material being sewn is moving from this direction toward the sewing needle, i.e., the material being sewn must always be turned into the corresponding direction in the case of curved or angularly projecting or offset edges, which requires an expensive pivoting mechanism, as is provided in the sewing machine according to the above-mentioned U.S. Pat. No. 4,498,404. The prior-art sensor device is unsuitable for other sewing machines, which do not have such a mechanism, and operate with a two-dimensional XY displacement of the material being sewn instead.
In view of this problem, it has been suggested that the sensor device with its sensor line, the optical system and the corresponding illumination, be arranged movably, rather than being permanently installed, so that it will be able to move around along a circular path around the sewing needle, as is described in EP-OS 0,309,069. This system creates a circular scanning zone that concentrically surrounds the presser foot. The area imaged on the sensor line is always a straight line section which also represents a chord in the circular scanning zone and can be followed by moving the device around the sewing foot, so that an edge of a material being sewn can always be monitored in a forward-looking manner, regardless of the direction from which the material being sewn is moving toward the sewing needle. It is a disadvantage of this device that an expensive and bulky mechanism with drive motor and gear parts, whose inertia also prolongs the response time of the entire control system, is required for the necessary circular movement.
A device which operates without moving mechanical parts to scan the material being sewn around the sewing needle in a redetermined direction from it for edge-parallel sewing has been known from U.S. Pat. No. 3,385,245. This device has four photocells arranged in the presser foot, which are arranged all around the needle passage hole of the presser foot at uniform distance from it. Since the photocells are located immediately above the workpiece, edge scanning is possible only with transmitted light, so that the use of this device is limited to translucent materials. Processing of materials that are not transparent to light, e.g., leather, is not possible with the prior-art device, because edges on such materials can be recognized in incident light only.
For detecting edges with incident light, it is in many cases absolutely necessary to arrange the light-receiving elements of the sensor device at a certain distance from the surface of the material being sewn in order for space to be left for oblique illumination of the edge, because oblique illumination is necessary in most cases to make edges more clearly visible, especially if the piece of material being sewn that forms the edge has the same surface brightness as the substrate. To image the points to be scanned on the surface of the material being sewn from the said distance on the light-receiving elements of the sensor device, an optical system must be present in the space between the plane of the material being sewn and the light-receiving elements.